Electrical filters have long been used in the processing of electrical signals. In particular, such electrical filters are used to select desired electrical signal frequencies from an input signal by passing the desired signal frequencies, while blocking or attenuating other undesirable electrical signal frequencies. Filters may be classified in some general categories that include low-pass filters, high-pass filters, band-pass filters, and band-stop filters, indicative of the type of frequencies that are selectively passed by the filter. Further, filters can be classified by type, such as Butterworth, Chebyshev, Inverse Chebyshev, and Elliptic, indicative of the type of bandshape frequency response (frequency cutoff characteristics) the filter provides relative to the ideal frequency response.
The type of filter used often depends upon the intended use. In communications applications, band-pass filters are conventionally used in cellular base stations and other telecommunications equipment to filter out or block RF signals in all but one or more predefined bands. For example, such filters are typically used in a receiver front-end to filter out noise and other unwanted signals that would harm components of the receiver in the base station or telecommunications equipment. Placing a sharply defined band-pass filter directly at the receiver antenna input will often eliminate various adverse effects resulting from strong interfering signals at frequencies near the desired signal frequency. Because of the location of the filter at the receiver antenna input, the insertion loss must be very low so as to not degrade the noise figure. In most filter technologies, achieving a low insertion loss requires a corresponding compromise in filter steepness or selectivity.
In commercial telecommunications applications, it is often desirable to filter out the smallest possible pass band using narrow-band filters to enable a fixed frequency spectrum to be divided into the largest possible number of frequency bands, thereby increasing the actual number of users capable of being fit in the fixed spectrum. With the dramatic rise in wireless communications, such filtering should provide high degrees of both selectivity (the ability to distinguish between signals separated by small frequency differences) and sensitivity (the ability to receive weak signals) in an increasingly hostile frequency spectrum. Of most particular importance is the frequency ranges of 800-900 MHz range for analog cellular communications, and 1,800-2,200 MHz range for personal communication services (PCS).
Of particular interest to the present invention is the need for a high-quality factor Q (i.e., measure of the ability to store energy, and thus inversely related to its power dissipation or lossiness), low insertion loss, tunable filter in a wide range of microwave and RF applications, in both military (e.g., RADAR), communications, and electronic intelligence (ELINT), and the commercial fields, such as in various communications applications, including cellular. In many applications, a receiver filter must be tunable to either select a desired frequency or to trap an interfering signal frequency. Thus, the introduction of a linear, tunable, band-pass filter between the receiver antenna and the first non-linear element (typically a low-noise amplifier or mixer) in the receiver, offers substantial advantages in a wide range of RF microwave systems, providing that the insertion loss is very low.
For example, in commercial applications, the 1,800-2,200 MHz frequency range used by PCS can be divided into several narrower frequency bands (A-F bands), only a subset of which can be used by a telecommunications operator in any given area. Thus, it would be beneficial for base stations and hand-held units to be capable of being reconfigured to operate with any selected subset of these frequency bands. As another example, in RADAR systems, high amplitude interfering signals, either from “friendly” nearby sources, or from jammers, can desensitize receivers or intermodulate with high-amplitude clutter signal levels to give false target indications. Thus, in high-density signal environments, RADAR warning systems frequently become completely unusable, in which case, frequency hopping would be useful.
Microwave filters are generally built using two circuit building blocks: a plurality of resonators, which store energy very efficiently at one frequency, f.sub.0; and couplings, which couple electromagnetic energy between the resonators to form multiple stages or poles. For example, a four-pole filter may include four resonators. The strength of a given coupling is determined by its reactance (i.e., inductance and/or capacitance). The relative strengths of the couplings determine the filter shape, and the topology of the couplings determines whether the filter performs a band-pass or a band-stop function. The resonant frequency f.sub.0 is largely determined by the inductance and capacitance of the respective resonator. For conventional filter designs, the frequency at which the filter is active is determined by the resonant frequencies of the resonators that make up the filter. Each resonator must have very low internal resistance to enable the response of the filter to be sharp and highly selective for the reasons discussed above. This requirement for low resistance tends to drive the size and cost of the resonators for a given technology.
Typically, fixed frequency filters are designed to minimize the number of resonators required to achieve a certain shape as the size and cost of a conventional filter will increase linearly with the number of resonators required to realize it. As is the case for semiconductor devices, photolithographically defined filter structures (such as those in high-temperature superconductor (HTS), micro electro-mechanical systems (MEMS), and film bulk acoustic resonator (FBAR) filters are much less sensitive to this kind of size and cost scaling than conventional combline or dielectric filters.
The approaches used to design tunable filters today follow the same approach as described above with respect to fixed frequency filters. Thus, they lead to very efficient, effective, and simple circuits; i.e., they lead to the simplest circuit necessary to realize a given filter response. In prior art tuning techniques, all the resonant frequencies of the filter are adjusted to tune the filter's frequency. For example, if it is desired to increase the operating frequency band of the device by 50 MHz, all of the resonant frequencies of the narrow-band filter must be increased by 50 MHz. While this prior art technique has been generally successful in adjusting the frequency band, it inevitably introduces resistance into the resonators, thereby disadvantageously increasing the insertion loss of the filter.
Although HTS filters may be tuned without introducing significant resistance into the resonators by mechanically moving an HTS plate above each resonator in the filter to change its resonant frequency, such technique is inherently slow (on the order of seconds) and requires relative large three-dimensional tuning structures. Insertion loss can be reduced in so-called switched filter designs; however, these designs still introduce a substantial amount of loss between switching times and require additional resonators. For example, the insertion-loss of a filter system can be reduced, by providing two filters and a pair of single-pole double-throw (SP2T) switches to select between the filters, thus effectively reducing the tuning range requirement, but increasing the number of resonators by a factor of two and introducing loss from the switch. The loss of the filter system can further be reduced by introducing more switches and filters, but each additional filter will require the same number of resonators as the original filter and will introduce more loss from the required switches.
There, thus, remains a need to provide a band-pass filter that can be tuned quickly with a decreased insertion loss.